1. Field of the Invention
The present invention relates to bonding tools used for fine wire bonding. More particularly, the present invention relates to capillary bonding tools used for making conventional strength ball bonds while constraining and controlling the size of the finished ball bond.
2. Description of the Prior Art
Capillary wire bonding tools are well known in the prior art and are classified in International Class HOI 21/603 and in U.S. Class 228, Subclasses 1.4; 4.5; 180 and other subclasses.
Capillary wire bonding tools that are used in automatic ball thermosonic wire bonders have been standardized into families or groups designed for special applications or problems. Fine pitch bonding tools are especially designed to enable bonding of pads which are closely spaced one to another. Due to miniaturization of semiconductor chips such as computer chips, the lead out pads on the chip and the pitch or spacing of pads on chips are constantly growing smaller. Pad sizes of 3.0 mils square placed on 3.2 mil centers are now being bonded using 1.0 mil fine gold wire which is melted or flamed off to form a 1.6 to 1.7 mil diameter air ball which is mashed to form a first bond. The diameter after making the bond increases at least 10% so that the resulting bonded ball has an average diameter of 1.8 to 1.9 mils. The mashed ball bond is not symmetrical and the maximum diameter can easily exceed the size of the bonding pad. Since the bonding tool has some placement error offset from the center of the bonding pad, conventional ball bonding produces bonds which over flow the edges of a bonding pad and the bonded devices are rendered unacceptable.
In the prior art, the term constrained bond and constrained bonding generally referred to controlling the amount of mash-out or lateral extrusion of a ball during ball bonding. Heretofore, this broad terminology has also included reducing the diameter of the air ball so that the mashed and expanded ball is of lesser size than a conventional mashed ball. While it was known that the ball size ratio (BSR) could be reduced from about 2.4 times wire size to about 1.7, the bonded area and the shear strength of the bond was also diminished. Further, if the air ball was too small it would clog the hole in the capillary during a bonding operation.
U.S. Pat. No. 5,558,270, assigned to the same assignee as the present invention, shows and describes a bonding tool developed for producing unsymmetrical mashed balls and is incorporated by reference herein. This tool employs a non-symmetrical working face having an elongated chamfer recess defined by two chamfer diameters CD1 and CD2 and is designed for use in a rotary head wedge wire bonder. The elongated chamfer recess produces a mashed ball bond that is more narrow in width than in length, thus, is well suited for bonding state of art semiconductor chips.
It would be desirable to provide a fine pitch bonding tool that could produce a smaller symmetrical bonded ball having enhanced shear strength. Such a symmetrical tool would be used on conventional non-rotary head automatic gold wire bonding machines which operate at higher speeds than rotary head bonding machines but could also be used on rotary head bonding machines.